Abstract

The BET (bromodomain and extraterminal) protein family members including BRD4 bind to acetylated lysines on the histone proteins, help assemble transcriptional regulators at the target gene promoters and enhancers, and regulate the expression of important oncogenes, e.g., Myc and BCL-2. Here we determined the effects of the BET protein inhibitor JQ1 and/or histone deacetylase (HDAC) inhibitor panobinostat (PS) on cultured (JeKo1 and MO2058) and primary human MCL cells harvested from the excised MCL involved lymph nodes. Treatment with JQ1 (100 to 2000 nM), but not its inactive enantiomer (R-JQ1), dose-dependently increased the % of cells in the G1 phase while reducing the % of S phase cells, while concomitantly inducing apoptosis in the cultured (MO2058 > JeKo1) MCL cells. Treatment with JQ1 was also dose-dependently lethal against primary MCL cells. JQ1 treatment reduced binding of BRD4 and RNA polymerase II to the DNA of MYC, BCL2 and CDK6 promoter in JeKo1 and MO2058 cells. Total RNA from the untreated and JQ1-treated cells was used for the quantitative PCR analysis, which showed depletion of the mRNA of c-MYC, BCL2 and CDK6 genes in JQ1-treated cells. While it had no effect on acetylated histone H3 and BRD4, JQ1 treatment dose-dependently depleted the protein levels of MYC, BCL2, CDK6 and pSer2 RNA POL II, but induced the levels of p21, p27 and cleaved PARP in MCL cells. As compared to each agent alone, co-treatment with JQ1 (but not its inactive enantiomer, R-JQ1) and panobinostat (PS) synergistically induced apoptosis of the cultured and primary MCL cells (combination indices< 1.0 by isobologram analyses), but not of normal CD34+ hematopoietic progenitor cells. This was associated with greater attenuation of MYC, BCL2, MCL1 and cyclin D1, but increase in the levels of p21 and cleaved PARP in cultured MCL cells. Co-treatment with JQ1 and PS was also synergistically lethal against carfilzomib-resistant JeKo1 cells (> 10 fold resistant), which were isolated following selection under a continuous exposure to increasing levels of carfilzomib. Following the tail vein infusion and engraftment of JeKo1 cells (5 million cells/mouse) in the bone marrow and spleen of NOD/SCID mice, co-treatment with JQ1 (50 mg/kg/day, formulated in 10% 2-hydroxypropyl-β-cyclodextrin, administered IP) and PS (5 mg/kg, IP) versus treatment with vehicle control, or JQ1 or PS alone, resulted in significant in vivo attenuation of c-MYC, BCL-2 and cyclin D1 levels in the harvested MCL cells from the mice (p < 0.01). Collectively, these pre-clinical findings demonstrate that the combined treatment with BRD4 antagonist and pan-HDAC inhibitor is a synergistically effective epigenetic therapy targeted against human MCL cells, regardless of their sensitivity to proteasome inhibitors.